1. Field of the Invention
The present invention relates to a process for the preparation of alkoxysilanes.
2. Discussion of the Background
Alkoxysilanes and alkoxysilane-based compositions are used in many sectors: as adhesion promoters, crosslinking agents in polymers, as release agents, as additives in paints and coatings, for hydrophobicization of surfaces, inter alia for textiles, leather and, in particular, for the protection of buildings and facades, for the preservation of books, for particular modification of the properties of surfaces, such as coating of glass fibers or silanizing of fillers and pigments, and also for improving the theological properties of polymer dispersions and of emulsions, to name only a few examples.
Halogenosilanes and in particular, chlorosilanes, are as a rule employed for the preparation of alkoxysilanes. The reaction of a halogenosilane with an alcohol to give an alkoxysilane is known per se to the expert and is also called esterification. For example, a chloropropylmethyidimethoxysilane is obtained by reaction of a chloropropylmethyidichlorosilane with methanol, hydrogen chloride being liberated. Even after purification of the product by distillation, however, residual halogen contents, i.e., residual amounts of acidic or hydrolyzable chloride, can undesirably remain in the alkoxysilane.
Efforts are currently being made to prepare alkoxysilanes and products which include alkoxysilanes, as well as alkoxysilane-based compositions with the lowest possible halide content. Products with the lowest possible chloride content are particularly desired. Another quality feature is the color, wherein the lowest possible Gardner/APHA color numbers according to ISO 6271 are being sought.
The above-described reaction of a halogenosilane with an alcohol is carried out either discontinuously or continuously, the hydrogen halide that is formed being converted into the gas phase or remaining bonded in the liquid phase. Customary techniques for separating off the hydrogen halide by conversion into the gas phase are stripping and distillation, including reactive distillation. Such processes are described, inter alia, in DE-A 20 61 189, U.S. Pat. No. 4,298,753, DE-A 28 00 017, DE-A 24 09 731, DE-A 27 44 726, DE-A 32 36 628, DE-A 862 895 and DD 31 751. Substantial disadvantages of these processes for separating off hydrogen halide by conversion into the gas phase include: (1) the secondary reactions of the hydrogen halide with the alkoxysilanes produced to give siloxanes in accordance with the equation: ##STR1##
and (2) the limiting of the conversion of the halogenosilanes into alkoxysilanes by the thermodynamic equilibrium as a result of of the hydrogen halide present in the reaction mixture. This applies in particular if more than one halogen atom is to be replaced by an alkoxy radical. In such a case, non-alkoxylated and mono- and polyalkoxylated silanes are present side by side. In the reaction of trihalogenosilanes with alcohol, the equilibrium constants decrease from the first to the third stage. This means that in particular the third alkoxy group is difficult to introduce, or that third alkoxy groups introduced readily react with hydrogen halide to re-form a halogenosilane structure.
A desirable alkoxysilane is 3-chloropropyl-methyl-dimethoxysilane (CPMDMO), which is formed from 3-chloropropyl-methyl-dichlorosilane and methanol. Here also, the reaction product of the first stage, the monoalkoxychlorosilane, is favored over the dialkoxysilane. This is shown by a comparison of the equilibrium constants, that of the first stage being significantly greater than 1 at room temperature, but that of the second stage being only 0.4. According to conventional methods, the reaction is carried out discontinuously in a stirred tank reactor and the hydrogen halide formed is separated off from the reaction mixture by distillation. The yield is about 60%. It can be increased to 65% by addition of methanolic sodium methylate solution, but with the accompanying disadvantages that: (1) sodium chloride is obtained; and (2) the risk of a 3-methoxypropyl group being formed from the 3-chloropropyl group in a Williamson synthesis.
The abovementioned disadvantages of the processes in which the hydrogen halide formed is converted into the gas phase are therefore particularly serious because the passage of the hydrogen halide from the liquid phase into the gas phase takes place relatively slowly and the mass transfer of the hydrogen halide thus decisively determines the overall process. The equilibrium therefore shifts only comparatively slowly in favor of the desired alkoxylated products; and the hydrogen halide promotes the formation of siloxanes in accordance with the above equation. The hydrogen halide passes into the gas phase particularly slowly in the case of reactions with methanol, since it is readily soluble in methanol. As a result of the slow passage of the hydrogen halide into the gas phase, the yields of alkoxysilane are reduced.
Another technique, which has already been mentioned in connection with the synthesis of 3-chloropropylmethyidimethoxysilane, for removing the hydrogen halide formed from the reaction mixture is bonding thereof in the liquid phase by reaction with basic substances. If acid-binding agents are employed, for example ammonia or (tertiary) amines (DE-A 913 769, in which the removal of hydrogen chloride with ammonia is described) or alcoholates, however, stoichiometric amounts of salt are obtained, and these usually have to be separated off from the product at great expense.
The use of solvents in the removal of the hydrogen halide is also described. The solvents serve here to reduce the viscosity of the constantly single-phase reaction mixture (DE-A 20 61 189) or to lower the boiling point of the constantly single-phase reaction mixture (DE-A 28 00 017, DE-A 32 36 628).
On the other hand, it is known that it is advantageous to choose a reaction temperature of &lt;100.degree.C. in the reaction of the halogenosilane with the alcohol, so that undesirable side reactions to give siloxanes are avoided (DE-A 24 09 731). This requires a removal of hydrogen halide from a temperature level which is as low as possible, although higher temperatures would be more favorable for this removal.
The use of various reactors is described for the preparation of alkoxysilanes, i.e., stirred tanks (for example GB 674 137), tube reactors (DE-A 20 33 373), stirred tanks with a column (for example DE-A 32 36 628) and reaction distillation columns (for example U.S. Pat. No. 4,298,753).
Processes for removing the residual halogen content from alkoxysilanes by reaction or neutralization with alkali metal alcoholates, such as, for example, sodium methanolate, and separating off the salt formed in the process are also known from EP 0 282 846 A2 and EP 0 741 137 A1. Such neutralization processes have the disadvantage that considerable amounts of product are also decomposed in this reaction. By-products are, inter alia, siloxanes, tetraalkoxysilane, addition products, such as methoxyethyltrimethoxysilanes by reaction of sodium methanolate and vinyltrimethoxysilane, methoxypropyltrimethoxysilanes formed from sodium methanolate and chloropropyltrimethoxysilane and methoxypropylmethyidimethoxysilanes from sodium methanolate and chloropropylmethyidimethoxysilane, or else, in the case of a reaction of alkali metal methanolate and an alkyltrimethoxysilane, highly toxic tetramethoxysilane.
Moreover, if customary neutralizing agents such as alkali metal alcoholates are used, a deterioration in the color number of the alkoxysilane treated is often observed.
The specifications CN 11 07 85 1A and 11 07 85 2A describe processes for the preparation of vinyltriethoxysilane and chloropropyltrimethoxysilane, which are said to give very pure products by the use of magnesium alcoholates. It is a disadvantage here that these alcoholates are difficult to handle since they are very sensitive to moisture. Furthermore, magnesium alcoholates are in powder form and tend to form lumps, which in turn makes accurate metering of this substance difficult. This disadvantage particularly relates to the metering into heated or hot products, which is usually carried out in practice, because the silane or other volatile constituents of the reaction formulation condense on the magnesium alcoholates, incipiently dissolve them and therefore convert them into a form which is no longer free-flowing.
To avoid this disadvantage, attempts have been made to employ alcoholic solutions of said alkaline earth metal alcoholates. The undesirable result, however, is that considerable amounts of alcohol are introduced into the neutralization step and must be separated off again by distillation, consuming time and energy. Furthermore, the magnesium alcoholates are expensive and are available in industrial quantities to only a limited extent.
The German Patent Application 198 49 196.4, which has not yet been published, discloses a process for neutralizing and decreasing the residual halogen content in alkoxysilanes or in alkoxysilane-based compositions, an alkoxysilane which has a residual content of acidic halide being subsequently treated with metallic magnesium in the presence of an alcohol. Such additional measures for after-treatment of products are as a rule complex and necessitate additional costs. It has furthermore been found in the reaction of chlorosilanes with alcohol that the alcohol reacts with the hydrogen chloride liberated and siloxanes are formed with progressive esterification. A high siloxane content is also to be recorded at an increased hydrogen chloride solubility in the system. These side reactions, which take place before the concluding neutralization step, have hitherto been controllable only with difficulty and lead to moderate yields because of the low selectivity.